Metallic nano-optic lenses and beam shaping devices

ABSTRACT

A nano-optic device comprises a plurality of subwavelength apertures in a metal film or between metal islands. The device is adapted to shape a radiation beam transmitted there through. For example, beam shaping includes at least one of beam focusing, beam bending and beam collimating.

The present application claims benefit of U.S. provisional applicationSer. No. 60/526,998, filed Dec. 5, 2003, which is incorporated herein byreference in its entirety.

The U.S. government may have certain rights in this invention pursuantto grant number 00014-99-0663 from the Office of Naval Research.

FIELD OF THE INVENTION

The present invention is directed generally to optical devices and moreparticularly to nanostructured optical devices and methods of making thedevices.

BACKGROUND

Beam shaping is an important concept in optics, and is commonly involvedin a variety of optical components and instruments. Focusing an opticalbeam through dielectric lenses is a good example, and their operation iswell understood on the basis of classical optics, i.e., curved surfaceswith an index contrast provide refraction and focusing of light. Interms of wave optics, the optical field emanating from a lens can beexpressed as Fourier expansion of radiation from infinitesimal dipoleson an exit surface of the lens. The role of the lens is to provide aphase correction to each of the Fourier components by virtue of phaseretardation resulting from path length difference. The optical fieldsthen reassemble to a focus at some distance beyond the lens, forming animage of the dipole source.

SUMMARY

A nano-optic device comprises a plurality of subwavelength apertures ina metal film or between metal islands. The device is adapted to shape aradiation beam transmitted there through.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simulation of optical transmission through a singlenanoaperture formed in a silver layer.

FIG. 2A is a side cross-sectional view of a device comprising athree-aperture array structure with tapered metal thickness according toan embodiment of the invention.

FIG. 2B is a simulation of beam propagation through the device of FIG.2A. The beam in FIG. 2B refracts towards the thicker metal side, similarto the case of conventional dielectric lenses. The dielectric in theapertures is assumed to be air in the simulation.

FIG. 3A is a side cross-sectional view of a device comprising anano-apertured metal lens. The lens is a five-aperture array that has aconvex profile of metal thickness.

FIGS. 3B, 3C and 3D are simulations of beam propagation through variousnano-aperture devices. FIG. 3B shows the focusing and collimation of abeam (650 nm wavelength) incident to the five-aperture lens of FIG. 3A.FIG. 3C shows focusing of a beam (650 nm wavelength) incident to athree-aperture lens. Comparison of FIG. 3C with FIG. 3B shows that theradiation components from the two outermost apertures also contribute tobeam shaping. FIG. 3D show beam focusing of a 800 nm wavelength beamincident on the five-aperture array lens of FIG. 3A.

FIG. 3E is a plot of a complex index n₁ versus beam wavelength for a 40nm aperture slit (dashed line) and 80 nm aperture slit (solid line).

FIG. 3F is a plot of transmittance versus slit depth.

FIG. 3G is a plot of phase versus slit depth.

FIG. 4A is a side cross-sectional view of a prior art micron-scaledielectric lens.

FIG. 4B is a simulation of beam propagation through the lens of FIG. 4A.As shown in FIG. 4B, a complex beam pattern develops due to a strongdiffraction effect at the lens edges.

FIG. 5 illustrates a schematic side cross-sectional view of a method offorming a non-uniform thickness profile of metal on nanoaperture (holeor slit) arrays.

FIGS. 6A and 6B illustrate side cross-sectional views of devicesaccording to an embodiment of the invention in which a micron-sizedielectric lens attached to a nanoaperture array.

FIG. 7 illustrates a side cross-sectional view of a device according toan embodiment of the invention in which the nanoapertures are filledwith dielectric materials having different refractive indexes.

FIG. 8 is a simulation of beam propagation through the device of FIG. 7.

FIG. 9 illustrates a side cross-sectional view of a device according toan embodiment of the invention in which the aperture width (or diameter)is modulated such that the effective refractive index can be properlyaltered, while keeping the metal thickness and aperture period constantacross the transverse directions.

FIG. 10 illustrates a side cross-sectional view of a device according toan embodiment of the invention in which metal islands are separated bynano apertures having a different depth or length.

FIGS. 11, 15 and 17 are side cross-sectional views of devices accordingto embodiments of the invention.

FIGS. 12, 13, 15 and 16 are top views of devices according toembodiments of the invention.

FIG. 18 is a schematic side cross-sectional view of a method of making adevice of FIG. 17.

FIGS. 19A and 19B are schematic side cross sectional views of a methodof making a device according to embodiments of the invention and FIG.19C is a schematic top view of a holographic lithography system.

FIGS. 19D-19I are schematic three dimensional views of a method ofmaking a device according to embodiments of the invention.

FIGS. 20A, 20B and 20C are micrographs of a method of making a nanoporearray according to embodiments of the invention.

FIG. 20D is a schematic side cross-sectional view of a device accordingto embodiments of the invention.

FIG. 20E is a schematic side cross-sectional view of an electroplatingbath used to make the device of FIG. 20D.

FIGS. 21A, 21B, 21C and 21D are schematic side-cross sectional views ofa method of making a device according to embodiments of the invention.

FIGS. 22A and 22B are schematic side cross-sectional views of a methodof making a device according to embodiments of the invention.

FIGS. 23A, 23B, and 23C are schematic side cross-sectional views of amethod of making a device according to embodiments of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A metallic nano-optic device possesses multifunctional capability inshaping and processing (i.e., focusing, bending, collimating, and/orspatial- and wavelength-filtering) an optical beam (i.e., a focused,unfocused or diffuse incident radiation) in a fashion that overcomes thelimitations of diffractive optics. The structure comprises ananoaperture array which is designed to transmit a light with properphase retardation between aperture elements such that the emerging beamevolves into a desired shape as it emanates from the apertures, similarto the beam shaping with a phased-array antenna in microwaves. As usedherein, the term light includes visible, ultraviolet and infraredradiation. The device utilizes the plasmonic phenomena occurring innanoaperture arrays and preferably has spatial-, wavelength-, and/orpolarization-filtering characteristic besides the beam shaping function.Finite-difference time-domain (FDTD) analysis results show that suchdevices are feasible in the optical frequency range. Methods of makingthe device are also described.

Preferably, optical beam shaping with the metallic nanoaperture arraystructures occurs when each nanoaperture serves as a dipole sourceradiating optical power at the exit surface of the metal. The dipoleelements in the array are designed to have a certain phase relationshipamong them, primarily controlled by the path length difference and/orthe effective index difference in the aperture regions. In the case ofdielectric-based optical lenses, the optical field is expressed as aFourier expansion involving an integral of continuously-distributedinfinitesimal dipole sources on the lens surface. In contrast, theoptical fields in the metallic nanoaperture lenses are expressed as adiscrete sum of finite dipole sources. As is shown below, this metallicnano-optic structure offers unique capability in shaping and processingoptical beams that are not obtained in the dielectric-based opticallenses.

Without wishing to be bound by a particular theory, the inventorsbelieve that one of the well-known limitations in dielectric lenses, forexample, is that no lens can focus light onto a size smaller than itswavelength. This can be understood in view of the fact that theinfinitesimal dipole elements located distant from a lens axis do notcontribute to the Fourier integral at a focus since the wave componentsemanating from those sources toward a focal point are non-propagatingand decay exponentially due to the large values of their transverse wavevectors. In the metallic nano-optic lenses, however, the situation isquite different. Being discrete and finite, the dipole elements radiateoptical power uniformly all around the radial directions. In otherwords, all of the nanoaperture dipoles in an array can generatepropagating waves that can reach the focal point with proper phaseretardation, thus contributing to image formation at a focus. Thisunique feature may allow the development of a novel beam-shapingmethodology that overcomes the limitations of diffractive optics. Forexample, it may be possible to design a nanoaperture array structuresuch that the radiations from a discrete set of finite dipoles focusesto a size significantly smaller than the wavelength, i.e., far below thediffraction limit. It should be mentioned here that the method to make a“perfect” lens described herein is different from the one proposed by J.B. Pendry involving negative refractive index material. J. B. Pendry,Phys. Rev. Lett. 85: 3966 (2000).

Another distinctive feature of the nano-optic lenses stems from theblocking nature of metal in optical transmission. The maximum lateraldimension of a transmitted beam is basically determined by the aperturearray dimension, and the transmitted beam profile is completely freefrom the diffraction effect regardless to the array size, even down tothe subwavelength scale. In the dielectric lenses, however, the lenssize may seriously interfere with and affect the transmitted beamprofile via diffraction at the lens edges, especially when the lensdimension is reduced to a size comparable to the wavelength of light.This diffraction phenomenon is one of the limiting factors in scalingdown the dimensions of conventional optics to wavelength orsubwavelength ranges. The metallic nanoaperture array structuresovercome these limitations, and preferably provide beam shaping atlength scales down to nanometers in a discrete or array form. Besidesthe above described features, the nano-optic structures also offer otherfunctionality such as wavelength- and/or polarization-filtering that areintrinsic to the structure. All these features provide multifunctionalbeam shaping and processing devices as described below.

FIG. 1 shows a FDTD simulation of optical transmission through a singlenanoaperture formed on a metal layer; the image shows the distributionof magnetic field (Hz). A single slit (80-nm slit width) is provided ina 200-nm-thick silver layer. An optical beam (633 nm wavelength) isincident from the bottom side in the image. The wavefronts of anincident and transmitted beam are clearly resolved with a period thatmatches the wavelength of light in the air and in quartz, respectively.The incident light transmits through the narrow slit, although theintensity drops significantly. The wavefronts emanating from the slitare clear concentric circles with uniform intensity distribution across0-180 degree angular range. This reveals that the nanoaperture on ametal layer serves as a finite dipole source and radiates power, in away similar to the case of a point (or line) source in free space. Itshould be noted here that the slit width is about ⅛ of the wavelength.As the slit width is increased, the transmission also increases, butwith a significant change in the transmitted beam profile. The angularuniformity of optical power deteriorates and a complex diffractionpattern develops.

FIG. 2A shows a cross-section of a three-aperture array structure 110,in which each nanoaperture 117 has different length (i.e., depth). Threeslits (80-nm wide (“SW”)) are introduced on a Ag layer with a 370-nmspacing (“d,” center to center). The Ag layer thickness is varied with a50-nm step profile (“dh”) such that the slit depth is 250, 300, or 350nm. The different slit depth is used to introduce phase retardationamong the dipole elements at the exit surface of metal. While FIG. 2Ashows a silver layer on a quartz substrate, other suitable metal andtransparent substrate materials may be used. Alternatively, thesubstrate may be omitted and a free standing metal film may be used.FIG. 2B shows a FDTD simulation of optical transmission through the slitarray. The image shows the optical intensity distribution. An opticalbeam (plane wave) is incident from the bottom side of the figure.Optical transmission through nanoaperture arrays involves interactionswith surface plasmons, and the transmittance depends on various factorssuch as slit spacing with respect to wavelength, refractive index ofsurrounding media, metal thickness, aperture width, etc. See Z. Sun, Y.S. Jung and H. K. Kim, Applied Physics Letters 83, 3021 (2003), and U.S.Provisional Application Nos. 60/492,954, 60/492,955 and 60/492,956,filed on Aug. 6, 2003, and incorporated herein by reference in theirentirety. The wavelength of light is chosen to match these structuralparameters so that a good transmission is observed (or vice versa, thestructural parameters can be designed to match a given wavelength formaximum transmission). The image clearly reveals that the transmittedbeam propagates along the direction tilted towards the thicker side ofmetal. This behavior is very much similar to the refraction of light inclassical optics. This can be understood in view of the fact that theeffective refractive index in a narrow slitted metal region is higherthan that in the air. Therefore the light emerging from the nanoslittedmetal region refracts (bends) towards the higher index region (metalside), in the same way as a light refracts when it exits from a higherindex region at a dielectric interface in ray optics. This simulationdemonstrates the feasibility of beam shaping/steering with the use ofthe nanoaperture arrays, whose geometry is properly shaped to induce aphase correction at the exit surface.

Based on this result, a lens structure 210 that has a convex profile inits metal thickness (aperture depth) is shown in FIG. 3A. Five slits 217(80 nm slit width “SW”) are provided in an Ag layer with 400-nm spacing(center to center). The metal thickness (slit depth) in the lens regionis designed to vary in a half-elliptical profile: the slit depth in thearray is 250, 320, 400, 320, and 250 nm. The diameter of the convexregion 219 is 2 microns (the diameter of the aperture array is 1.7microns). FIG. 3B shows a FDTD simulation of optical transmission (at650 nm wavelength) through the nano-optic lens structure (i.e., an imageof the optical intensity distribution). The image shows that theincident beam is well focused and collimated after the nanoaperturelens. The beam size (full-width-half-maximum) is measured to be ˜700 nm,about the same as the wavelength of light. It should also be noted thatthe beam remains well collimated with negligible divergence even aftermany wavelengths of propagation in the far field regime.

In order to check the contributions from each aperture (especially fromthe outer ones) in this beam shaping, the two outermost apertures in thefive-aperture lens structure were deleted in the simulation. FIG. 3Cshows the optical transmission through a three-aperture lens structureat 650 nm wavelength. The focusing effect became stronger while thecollimation effect was lost. This simulation demonstrates that theradiation components from the outer apertures also reach the far-fieldregion and contribute to beam shaping with proper phase retardation,whose amount is basically controlled by the lens thickness in thisparticular design.

Changing the operating wavelength is expected to alter the phaseretardation among the nanoapertures. FIG. 3D shows a FDTD simulationwith the five-aperture lens at 800 nm wavelength. Compared to the caseof operation at 650 nm wavelength (shown in FIG. 3B), the transmittedbeam shows a more focused profile with a minimum beam waist of ˜400 nm.The change in phase retardation among the apertures is visuallyconfirmed in the beam propagation characteristic in the apertureregions. The fringe spacing in each nanoslit, for example, is basicallydetermined by the effective refractive index and the wavelength.Comparison of FIGS. 3B and 3D reveals that the fringe spacing at 800 nmwavelength is noticeably larger than that at 650 nm, implying that thephase retardation between neighboring apertures decreased at longerwavelength. The fringe spacing in the nanoaperture regions is smallerthan that in air region at each wavelength. This confirms that theeffective refractive index in the nanoaperture region is higher thanthat in the air, as expected from an understanding of the plasmonphenomena in nanostructures. Shifting the operating wavelength will notonly change the phase relationship between apertures but also will alterthe transmitted power through each aperture. See Z. Sun, Y. S. Jung andH. K. Kim, Appl. Phys. Lett. 83, 3021 (2003).

The dependence of optical transmission (magnitude and phase) on thewavelength and structure may be taken into account in designing thenano-optic lens structures that produce desired beam profiles. Theoptical fields (the magnetic field H_(z)) in the far-field regime of abeam transmitted through the nanoslit array can be expressed as asummation of the cylindrical waves from each nanoslit element.

$\begin{matrix}{{H_{z}\left( {x,y} \right)} = {\sum\limits_{\alpha}{\frac{A_{\alpha}}{\sqrt{r_{\alpha}}}{\mathbb{e}}^{{\mathbb{i}\phi}_{\alpha}}{\mathbb{e}}^{{\mathbb{i}}\; k_{o}r_{\alpha}}}}} & (1)\end{matrix}$

Here, r_(α)=√{square root over ((x−x_(α))²+(y−y_(α))²)}{square root over((x−x_(α))²+(y−y_(α))²)}, and k₀ is the wavevector of the transmittedbeam in the air region. A_(α) and φ_(α) are respectively the amplitudeand phase of the radiation component emanating from the α-th slitlocated at (x_(α),y_(α)), and are expressed as follows, if we neglectcoupling between the slits.

$\begin{matrix}{A_{\alpha} = {\frac{\tau_{\alpha 01}\tau_{\alpha 12}{\mathbb{e}}^{{\mathbb{i}}\; k_{\alpha}h_{\alpha}}}{1 + {\rho_{\alpha 01}\rho_{\alpha 12}{\mathbb{e}}^{{\mathbb{i}2}\; k_{\alpha}h_{\alpha}}}}}} & (2) \\{\phi_{\alpha} = {\phi_{\alpha 01} + \phi_{\alpha 12} + {n_{1}k_{0}h_{\alpha}} - \theta_{\alpha}}} & (3)\end{matrix}$

Here, ρ_(α01) and ρ_(α12) are the reflectivity of surface plasmon waveat the air-metal and metal-substrate interfaces of the α-th slit,respectively, and are given as ρ_(α01)=(n₀−n_(1α))/(n₀+n_(1α)) andρ_(α12)=(n_(1α)−n₂)/(n_(1α)+n₂). φ_(α01)=arg(ρ_(α01)) andφ_(α12)=arg(ρ_(α12)). τ_(α01)=1−ρ_(α01) and τ_(α12)=1−ρ_(α12). Thecomplex refractive index n_(1α) relates the surface plasmon wavevectork_(α) in the α-th slit region to the wavevector in the air region(k_(α)=n_(1α)k₀), and n₀ and n₁ are the refractive indices in the airand dielectric substrate, respectively.θ_(α)=arg(1+ρ_(α01)ρ_(α12)e^(i2k) ^(α) ^(h) ^(α) ). h_(α) is the depthof the α-th slit. In general, both the amplitude (A_(α)) and phase(φ_(α)) are complex functions of the structural and materials parameters(such as slit width, depth and spacing, and dielectric constants) andthe operating wavelength relative to slit spacing.

In order to attain a quantitative understanding, the inventorscalculated the surface plasmon wavevector in the nanoslit region andthus the complex index n₁ (FIG. 3E). For a 80-nm-wide slit formed insilver, n₁ is calculated to be 1.3+i0.01 at 650 nm wavelength. As theslit width is reduced, both the real and imaginary parts of index n₁increase, indicating that the portion of the surface plasmon field inthe metal region grows. In the regime that no resonance coupling occursbetween surface plasmon waves localized at each slit, both the amplitude(A_(α)) and phase (φ_(α)) of optical field are primarily determined byslit depth (i.e., metal thickness). FIGS. 3F and 3G show slit depth(i.e., thickness) dependences calculated using the formula describedabove. In the case of 80-nm-wide slits, the amplitude is found to remainnearly constant over a broad depth (thickness) range, i.e., with amaximum variation of 0.91 to 0.98 for the metal depth (thickness) of 100to 1000 nm. The periodic fluctuation of amplitude indicates theFabry-Perot resonance of surface plasmon wave in the nanoslit region.The calculation also shows that the phase of optical field is linearlyproportional to slit depth. In this regime (a uniform power distributionamong slits and the linear dependence of phase on slit depth), it can beshown that the transmitted waves through the nanoslits will beam intothe direction that satisfies the following phase matching condition atthe metal/air interface: k_(sp) sinθ_(i)=k₀ sinθ_(t). Here θ_(i) is theincidence angle of the surface plasmon wave to the hypothetical planarsurface that comprises the slit apertures, and θ_(t) is the tilt angleof the transmitted beam. This formula basically tells that light willrefract at the nanoapertured metal surface in the same manner as in thecase of dielectric interfaces. It should be noted that the slit spacingin this design is smaller than the wavelength of light. Therefore, nograting diffraction effect is involved in the optical transmissionthrough the nano-optic structure, unlike the diffractive optics case.The FDTD simulation result demonstrates that the nanoslit arrays withtapered metal thickness possess the capability of beam shaping in a waythat resembles the dielectric-based refractive optics (i.e., refractionthrough curved surfaces) but that is distinctively different from theconventional optics in its mechanism (i.e., transmission of opticalpower through metal via the surface plasmon waves propagating throughthe nanoslit arrays).

In order to compare the beam shaping performance of the nanoaperturearray device with that of conventional dielectric lenses, the inventorssimulated the optical transmission through a dielectric lens structure.FIG. 4A shows a cross-section of a glass lens 310, whose dimension isapproximately the same as the nanoaperture array shown in FIG. 3A: 2micron lens width (diameter) and 600 nm lens height (the thickness atcenter). A beam (650 nm wavelength) is incident from the bottom side inthe image shown in FIG. 4B. The transmitted beam shows a strongdiffraction effect at the lens edges, although a focusing effect is alsoevident in the center region. The edge diffraction effect becomes evenmore serious as the lens width is further reduced to a wavelength orsmaller. This is a big contrast to the nanoaperture lens case, in whichthe edge effect is intrinsically absent due to the blocking nature ofmetal in the outside the array region and also due to the discretedipole (point source like) nature of radiation from the nanoapertures.

The beam-shaping functions of the nanoaperture array device may be usedin any suitable fields or devices that involve optics.

For example, the capability of focusing an image into a size muchsmaller than the wavelength of light means that the device may be usedin optical lithography and patterning and imaging beyond the diffractiveoptics. In other words, the nano-optic beam shaping devices, such as thefocusing and/or collimating lenses described herein, are incorporatedinto a lithography exposure system to focus the exposing beam, such as avisible or UV radiation beam, onto a photosensitive layer, such as apositive or negative photoresist layer. The radiation beam mayoptionally be passed through a photo mask before being incident onto thenano-optic beam shaping device. The exposed photosensitive layer is thenpatterned to form a mask, and then the device layer(s) underlying themask is wet or dry etched to form a corresponding pattern in theunderlying layer or layers. Thus, any suitable device, such as asemiconductor or other solid state device, such as transistors,capacitors, fuses, etc. may be patterned using this lithography method.Any one or more layers, such as semiconductor, conductive (i.e., metalor polysilicon) or insulating layers, in these devices may be patternedusing this method.

The capability of beam shaping and collimating of an optical beam atwavelength or subwavelength scale without being affected by the edgediffraction effect means that the device can also be used in opticalinstrumentation that involves beam coupling and light detection incompact space (both in the beam propagation direction and the transversedirection) and in scaleable large size arrays. The focused beam spotsize may range from 10 nm to 800 nm, such as 100 nm to 750 nm or 10 nmto 100 nm.

The nanoslit arrays (i.e., arrays with slit shaped apertures) also showstrong polarization and wavelength dependence in optical transmission,and this can be utilized as a polarization and wavelength filter as anintegral part of the lens, collimator or beam bending device, as will bedescribed in more detail below.

The nano-optic beam shaping/lensing concept can be extended to the 2Dnanoaperture (hole) array structures, although the FDTD simulationsshown in the figures were carried out for the 1D arrays forcomputational convenience. In the 1D nanoslit arrays described above,beam shaping (focusing for example) is designed to occur along thedirection parallel to the grating vector direction. In other words,focusing occurs along the polarization direction. In alternative designsof lens structure, beam shaping can be designed to occur along thedirection perpendicular to the polarization direction. This latterconfiguration will be useful for controlling fast-axis divergence oflaser diode beams, in which case the laser beam is usually TE polarizedand the beam diverges fast along the direction perpendicular to thepolarization direction.

Overall the proposed nanoaperture structures are promising fordeveloping ultracompact, multifunctional optical components andinstruments. The nanoaperture structures (non-uniform thickness profilesof metal) can be implemented by combining a microfabrication processwith the nanofabrication methods described by H. K. Kim et al., U.S.Provisional Application Nos. 60/492,954, 60/492,955 and 60/492,956 filedon Aug. 6, 2003 and incorporated herein by reference in their entirety.

FIG. 5 below illustrates a schematic method of forming a nonuniformthickness profile of metal on nanoaperture (hole or slit) arrays. Ashadow mask 502 with micron scale apertures 504 is placed on top of aflat nanoaperture array substrate, such as a metal sheet of film 505.Deposition of metal 508 on the substrate 505 is performed using theshadow mask 502. Due to the finite thickness of a mask, the metaldeposited through the apertures will show nonuniform, usually convex,thickness profiles 509. The convex metal portions are preferably, butnot necessarily, deposited over a flat metal film substrate 505. Afterthe shadow mask is removed, the nanoapertures 507 are formed in themetal by photolithography or other suitable methods, as described inmore detail below.

FIG. 6A illustrates an alternative embodiment of a device 601 where amicron-size dielectric lens 609, such as a glass, plastic, epoxy, quartzor transparent ceramic lens, is attached to a nanoaperture array 605that preferably, but not necessarily, has a flat or uniform metalthickness. The lens may be attached to the metal film 605 by anysuitable method, such as by a transparent adhesive, by mechanicalattachments, by compression bonding and/or thermal bonding. The phasecorrection function is performed by the dielectric lens part while thenanoaperture array provides a discrete set of dipoles that radiate poweras a point (or line) source. The light can be incident from the metalside as shown in FIG. 6A or from the dielectric lens side as shown inFIG. 6B. The latter configuration is expected to perform more closely tothe nanoaperture lens concept described herein.

FIG. 7 shows an alternative device 701 where the inside of thenanoapertures 707 are filled with dielectric materials 708A, 708B, 708Cwhose refractive indices are properly modulated such that proper phaseretardation results during transmission through the apertures. In otherwords, the apertures are filled with dielectric materials havingdifferent refractive indexes. Alternatively or in addition, a layer (orlayers) of dielectric with spatially-modulated dielectric constant canbe placed on top and/or bottom surface of nanoaperture array 705. Inother words, a layer of dielectric contains regions of differentrefractive index overlying different apertures 707. In the FDTDsimulation shown in FIG. 8, the refractive index of materials 708A,708B, 708C in the apertures is assumed to be 1.2, 1.6 and 2.0respectively from the left. Note that the transmitted beam refractstowards higher index region. In an alternative aspect of the invention,one or more materials whose dielectric properties, such as therefractive index value, are tunable with an external field (voltage)applied across the dielectric region, are provided in the apertures 707and/or as one or more layers above and/or below the apertures 707. Thephase retardation through each aperture can then be changed bycontrolling the voltages, and variable/tunable beam shaping functionscan be achieved without altering the geometry of the aperture arraystructure. Besides the time domain modulation, dielectric properties canbe modulated in a spatial domain utilizing the intensity dependent indexchange (such as the case of photorefractive effects) of certaindielectric materials. This can be utilized in defining lens profiles(index profiles and thus beam shaping functions) in a programmablefashion. For example, tunable dielectric materials, such as nematicliquid crystal materials whose refractive index and dielectric constantare changed by an applied voltage, and control methods described in U.S.published application No. 20030128949, incorporated herein by referencein its entirety, may be used.

Different refractive index materials may be incorporated into differentapertures by a lift-off method which includes selectively masking somebut not all apertures by a first photoresist layer, depositing a firstrefractive index material into the exposed apertures and over the firstphotoresist layer, lifting off the first photoresist layer, forming asecond photoresist layer masking the filled apertures, depositing adifferent second refractive index material into the exposed aperturesand over the second photoresist layer, and lifting off the secondphotoresist layer. Alternatively, the photoresist layers may be used asa mask to selective etch the different refractive index materials outfrom different apertures rather than for a lift off method.

FIG. 9 shows an alternative structure 901 where the aperture 907 width(or diameter) in the metal film 905 is modulated such that the effectiverefractive index can be properly altered, while keeping the metalthickness and aperture period constant across the transverse directions.This design utilizes the plasmon characteristic in a nanoaperturestructure that the smaller the aperture width, the higher the effectiverefractive index in the aperture region.

FIG. 10 below shows an alternative structure 1001 where metal islands1005 on a transparent substrate 1003 are separated by nanoapertures 1007having a different depth or length. To form this structure, first, atrench (or a hole) profile of different etch depths is developed on atransparent dielectric substrate 1003. A metal coating 1005 is providedon the surface and side walls of the dielectric trenches by any suitablemethod such as angled deposition, as described in more detail below.

In another alternative configuration, the beam shaping nano-optic devicestructure is a combination of structures shown in FIGS. 2, 3, 6, 7, 9and/or 10. In other words, any two or more of different depth aperturesin metal islands or metal film, a dielectric lens above the apertures,different dielectric constant materials in and/or above differentapertures and different width apertures may be used in the same deviceto enhance the beam shaping characteristics of the device.

The nano-optic structure preferably includes a metal film or a pluralityof metal islands, having a plurality of openings or apertures, eachopening or aperture having a width that is less than a first peakwavelength of incident radiation to be provided onto the film orislands. It should be noted that the term “each aperture” does notpreclude the structure from including other openings or apertures havinga width that is greater than the first peak wavelength. The metal filmor islands are configured such that the incident radiation is resonantwith at least one plasmon mode on the metal film or metal islands,thereby enhancing transmission of radiation having at least one secondpeak wavelength through the openings or apertures. For incidentradiation having multiple peak wavelengths, the first peak wavelength isthe median peak wavelength. The nano-optic structure preferablycomprises a lens and a method of focusing a radiation beam comprisespassing the radiation beam through the lens comprised of a metal film ora plurality of metal islands having a plurality of apertures, eachaperture having a width that is less than a peak wavelength of theincident radiation, such that the beam is focused to a spot size that isthe same as or smaller than the peak wavelength of the radiation beam.

The beam shaping nano-optic device described above may also comprise awavelength separation device or filter or it may be used in combinationwith a nano-optic wavelength separation device described below. FIG. 11is schematic illustration of wavelength separation using a stacked onedimensional (1D) slit array as a micron-scale monochromator orwavelength separation device 101. FIG. 12 illustrates the top of thedevice 101. As shown in FIG. 11, incident radiation having a range ofwavelengths λ₁ to λ_(n) is provided onto a metal film 105 having aplurality of openings 107. The openings have a width that is less thanat least one wavelength of incident radiation, such that the incidentradiation is resonant with at least one plasmon mode on the metal film.The transmitted radiation is provided through the plurality of openingssuch that the transmitted radiation is simultaneously separated into aplurality of passbands having different peak wavelengths λ₁, λ_(j), andλ_(k). The incident radiation may be provided onto either side of thefilm 105.

The wavelength separation device 1, 11 201 (as described below) or 101may comprise the nano-optic beam shaping device by varying the depth ofthe apertures, by filling or covering different apertures with differentrefractive index transparent dielectric materials and/or by varying thewidth of the apertures. Alternatively, the wavelength separation devicemay be used in combination with the nano-optic beam shaping devicedescribed above, where the incident radiation is first passed throughdevice 1, 11, 101 or 201 and then the transmitted radiation is passedthrough the beam shaping device.

Preferably, radiation having a peak wavelength less than 700 run, suchas 400 nm to 700 nm (i.e., visible light) is used as the incidentradiation. In this case, the apertures or openings have a width of 700nm or less, such as 15 to 200 nm, preferably 40 to 60 nm. In the case ofincident radiation with longer wavelengths, such as infrared radiation,the openings may have a proportionally larger width.

In this device 101, a metal layer or film 105 is formed over a radiationtransparent substrate 103. However, a free standing metal membrane filmwithout a supporting substrate or metal islands on a substrate may beused instead. For example, FIG. 13 illustrates a wavelength separationdevice 1 containing metal islands 5 separated by transparent regions 7.

The metal film 105 contains slit shaped openings 107 that areperiodically arranged with a cellular pattern. The slits preferably havea length that is at least ten times larger than the width. However, theopenings 107 may have any other suitable shape, such as round, oval,polygonal or irregular shape. For example, FIG. 14 illustrates a device201 containing a metal film 205 with groups of round openings 207arranged in cells 208A, 208B and 208C.

The metal film 105 is divided into a desired number of cells or regions108, such as at least two cells, where the grating period of theopenings 107 is substantially the same within each cell. However, thegrating period of the openings 107 differs between cells. In otherwords, the openings 107 in each cell are spaced apart from adjacentopenings in the cell by about the same distance. However, this distanceis different for different cells. For example, three cells 108A, 108Band 108C are illustrated in FIG. 11.

The grating period of the openings 107 in each cell 108 is designed toproduce a passband at a certain peak wavelength in the transmissionspectrum. Thus, a transmission of the radiation having one peakwavelength is enhanced due to the period of the openings in the firstcell 108A. A transmission of the radiation having a different peakwavelength is enhanced due to the different period of the openings inthe second cell 108B.

Preferably, the device 101 contains at least ten cells, more preferablyat least 30 cells, such as 30 to 1,000 cells. A period of openings ineach of the cells is different than periods of openings in each of theother cells. The transmission of passband radiation having a differentpeak wavelength through openings in each cell is enhanced due to theperiod of the openings in the respective cell. Preferably, the passbandradiation transmitted through each cell 108 has a peak wavelength thatdiffers by at least 10 nm, such as 10 to 100 nm, from peak wavelengthsof radiation transmitted through the other cells 108.

The propagation length of surface plasmons is estimated to be about 5 toabout 10 microns. A cell size comparable to this number or larger ispreferred because it allows sufficient plasmon interaction. A 10-μmcell, for example, corresponds to about 30 periods of gratings when0.5-μm peak passband wavelength is assumed. The cell size may be greaterthan 10 microns, such as 10 to 10,000 microns, for example, and thenumber of gratings per cell varies by cell size and peak passbandwavelength.

A cell 108 size of about 10 microns, such as 5-20 microns is preferredbecause it matches a typical pixel size of commercially available CCDdevices. For high array density (i.e., for better spatial resolution),it is desirable to keep the cell size as small as possible. However, forease of fabrication, the cell size may be increased to about 50 to 500μm.

Preferably, a period of openings in each cell ranges from about 250 nmto about 700 nm and a width of each opening preferably ranges from about20 nm to about 80 nm for visible light incident radiation. The width ofthe openings 107 may be larger for infrared incident radiation.

An alternative design to the 1×N array pattern described above is toutilize a chirped grating (i.e., opening) pattern. In other words, thegrating period (i.e., the period of the openings) is continuouslychirped over a distance, L. If a radiation detector is used with thewavelength separation device, then the detector pixel size, W, definesthe effective cell size of a wavelength separation device, such as afilter, and the total number of channels of the array will be L/W. Anadvantage of this design is that an entire monochromator array can beimplemented with a single holographic lithography process, as will bedescribed below.

FIGS. 15 and 16 illustrate a wavelength separation device 11 accordingto a second preferred embodiment of the present invention. As discussedabove, the wavelength separation device 11 may also comprise the abovedescribed beam shaping device or be used together with the beam shapingdevice. In the second embodiment, the metal film or metal islands 15have a periodic or quasi-periodic surface topography 12 provided on atleast one surface of the metal film or islands 15, as shown in FIG. 15.If desired, the metal film or islands may be formed on a radiationtransparent substrate 13. The topography 12 is configured such that itenhances the transmission of the radiation in the openings 17. Theperiodic topography 12 may comprise any metal features which providestrong coupling of the metal surface plasmons with incident radiation.For example, the topography may comprise any suitable raised and/ordepressed regions on the surface of the metal film or islands 15 whichare arranged in a regularly repeating (i.e., quasi-periodic or periodic)pattern, such as a two dimensional lattice. The raised regions maycomprise cylindrical protrusions, semi-spherical protrusions, linear orcurved ribs, rectangular ribs, raised rings and/or raised spirals. Thedepressed regions may comprise cylindrical depressions, semi-sphericaldepressions, linear or curved troughs, rectangular troughs, ring shapedtroughs and/or spiral shaped troughs. The width or diameter of theraised or depressed regions is preferably less than the period of thesefeatures, and the product of this period with the refractive index ofthe substrate should be less than the maximum desired transmittedwavelength of the radiation.

The metal film or metal islands 15 comprise at least two cells 18, andpreferably a plurality of cells, such as at least 10 cells, morepreferably at least 30 cells. Each cell 18A, 18B, 18C, 18D comprises atleast one of a plurality of openings 17. The periodic or quasi-periodicsurface topography 12 configuration in each of the cells is differentthan periodic or quasi-periodic surface topography configurations ineach of the other cells. Each cell is configured for transmission ofpassband radiation having a different peak wavelength, as in the firstpreferred embodiment.

While the linear grating patterns illustrated in FIGS. 12-15 havepolarization detection capability as an intrinsic function, thepolarization dependence of filters may not be desirable for someapplications. The patterns illustrated in FIG. 16 are insensitive topolarization in its transmission. For example, as shown in FIG. 16,circular grating patterns 12 are used in forming corrugations ofconstant period for each concentric pattern. A subwavelength aperture 17is made at the center of each pattern, and the incident light will befunneled into the aperture via resonant excitation of surface plasmonsat a certain wavelength, which is determined by the grating period.Arranging the circular grating patterns of different periods into a twodimensional array, such as the 2×2 array shown in FIG. 16 results in a4-channel spectrum analyzer that is insensitive to polarization.

In another preferred aspect of the second embodiment, the surfacetopography 12 comprises a topography comprising a material other thanmetal which includes surface plasmon coupling into the metal. In oneexample, the refractive index of the dielectric layer or ambient mediumadjacent to the metal surface is periodically or quasi-periodicallymodulated, without topographic modulation of the metal surface (i.e.,without corrugation/indentation on the metal surface). For example,periodic arrangement of dielectric layer or layers formed on a flat orcorrugated metal surface can induce the surface plasmon coupling intometal. Thus, element 12 in FIG. 2D may refer to periodically orquasi-periodically arranged dielectric material features formed on aflat metal film or island 15 surface. Alternatively, a flat or textureddielectric layer or layers with a variable refractive index may be usedfor plasmon coupling. A variable refractive index in a flat dielectriclayer or layers may be achieved by periodically or quasi-periodicallymodulating the composition of the layer or layers along their width onthe metal film or islands. Any suitable dielectric material many beused, such as silica, quartz, alumnia, silicon nitride, etc.

In the second preferred embodiment, the openings or transparent regions17 are separated by a period a_(o) which is much larger than the periodof the first embodiment, such that the period of the openings 17 doesnot substantially contribute to the enhancement of the transmission ofthe radiation. For example, the period a_(o) is preferably equal to theeffective propagation distance of the surface plasmons, such as 5microns or greater, preferably about 5-10 microns for Ag islands beingirradiated with visible light.

The device 101 also acts as a polarization filter. In the case of slitshaped openings in the metal film or between metal islands, (i.e., a 1Dgrating case), the optical transmission through the sub-wavelengthopenings depends on the polarization of incident light. For the TEpolarized light (i.e., where the E-field is parallel to the gratinglines), for example, surface plasmons are not excited due to theunavailability of grating vectors along the E-field direction, since thesurface plasmons are longitudinal waves. Therefore, transmission for TEpolarizations is expected to be much lower than TM polarization. Thispolarization dependence can also be utilized for detecting thepolarization (and its spatial distribution) of incident light.

Any suitable metal such as Ag, Al, Au and Cu may be used to form themetal film or metal islands. Preferably, metals, including Ag, Al, Au,Cu or their alloys, which exhibit a bulk plasmon frequency in the 9-10eV range are used. This makes the plasmon-induced phenomena observablein a broad spectral range (Vis-to-IR). Al and Cu are common metals usedas interconnect metallization in integrated circuit chips andphotodetectors.

Any suitable methods of making the metal islands or metal film with nanoapertures may be used to form the nano-optic beam shaping device. Forexample, the methods described in U.S. Provisional application Nos.60/492,954, 60/492,955 and 60/492,956 filed on Aug. 6, 2003 andincorporated herein by reference in their entirety, may be used.

For example, if a metal film with apertures is used in the nano opticbeam shaping and/or radiation filtering device, then the apertures maybe formed by any suitable method. For example, the apertures may beselectively drilled in the metal film using a focused ion beam.Alternatively, the apertures may be formed by photolithography, where aphotoresist or other photosensitive layer is selectively exposed toradiation, either through a mask or by selectively moving the exposingelectron or radiation beam over the photoresist, the exposed photoresistis then patterned into a mask and the regions of the metal film exposedin the mask are etched by wet or dry etching.

For example, if metal islands are used in the nano optic beam shapingand/or radiation filtering device, then these islands may be formed byany suitable method. For example, in one preferred aspect of the presentinvention, the metal islands spaced apart by radiation transparentregions or slit shaped openings are formed by self assembly. In otherwords, rather than forming a metal film and patterning the film intometal islands, the spaced apart metal islands are formed simultaneouslyor sequentially without first being part of an unpatterned metal film.The metal islands may comprise discrete metal islands that are notconnected to each other (i.e., the metal islands are not in directcontact with each other) or metal islands that are connected to eachother at a peripheral region of the optical device. In another preferredaspect, the metal islands comprise discrete islands that are formed bypatterning a metal film into the islands. Preferably, the islands arepatterned using a lithographic method.

The metal islands 5 may have any suitable thickness such that theislands 5 themselves are opaque to radiation but will generate plasmonenhanced radiation transmission through openings or regions 7.Preferably, metal island thickness should be at least about two or threetimes the skin depth of metal. In silver islands with incident radiationin a visible wavelength range, the skin depth is around 30 nm, and themetal island thickness should be at least about 60 to 90 nm or greater.The skin depth increases for longer wavelength range and is somewhatdifferent for different metals. Thus, for example, the metal islands 5may have a thickness of about 50 nm to about 2000 nm, such as 100 nm to400 nm, preferably 120 to 180 nm.

In a preferred aspect of the first and second embodiments, the metalislands 5, 15 are formed by self assembly and are located on a pluralityof ridges 21 on the transparent substrate 3, 13. Preferably, as shown inFIG. 17, each one of the plurality of metal islands 5, 15 is located ona corresponding one of the plurality of ridges 21. The metal islands andthe ridges may have any suitable shape, as discussed above. Preferably,the metal islands and the ridges are shaped such that the openings 7, 17between the islands are slit shaped. Thus, a length of each metal islandis preferably at least 10 times larger than its width and a length ofeach ridge is preferably at least 10 times larger than its width. Asshown in FIG. 17, the plurality of ridges 21 preferably have arectangular shape. The ridges 21 may comprise protrusions on the upperportion of the radiation transparent substrate 3, 13, protrusions on theupper portion of a radiation transparent layer located on the radiationtransparent substrate or the photodetector 302, 402, or discreteradiation transparent elements located over the radiation transparentsubstrate or the photodetector 302, 402. Thus, the substrate 3, 13 maycomprise a unitary substrate (i.e., a single layer radiation transparentmaterial) or it may contain more than one layer of radiation transparentmaterial. The ridges 21 may have a variable period to form devices ofthe first preferred embodiment.

Preferably, each respective metal island 5, 15 extends over an uppersurface 23 of each ridge 21 and over at least a portion of at least oneside surface 25 of each respective ridge 21. Most preferably, the metalislands are formed by angled deposition, as shown in FIG. 18. In thiscase, each metal island 5, 15 extends lower over a first side surface 25of a respective ridge 21 than over a second side surface 27 of therespective ridge 21 because the metal is angle deposited from the firstside surface 25, as will be described in more detail below.

In an alternative aspect of the present invention, the substrate 3, 13comprises a nanopore array. Preferably, the substrate 3, 13 comprises ananodic aluminum oxide nanopore array located over a radiationtransparent substrate or the photodetector, as will be described in moredetail below.

The optical devices 1, 11 of the preferred aspects of the presentinvention may be made by any suitable method where a plurality of metalislands 5, 15 are formed on the radiation transparent substrate 3, 13.As described above, the metal islands 5, 15 are preferably selectivelydeposited on the plurality of ridges 21, such that metal is notdeposited between the ridges 21.

FIG. 18 illustrates a preferred method of selectively forming the metalislands 5, 15 by self assembly using angled deposition. In this method,the metal is directed onto the ridges 21 in a non perpendiculardirection with respect to tops of the ridges. For example, if the ridgescontain a flat upper surface 23, then the metal may be directed at anangle of 20 to 70 degrees, such as 30 to 50 degrees, with respect to theflat upper surfaces 23 of the ridges.

Preferably, the metal islands 5,15 are deposited on the ridges 21 byevaporation (thermal or electron beam), as shown in FIG. 18. In theevaporation method, the metal is evaporated thermally or by an electronbeam from a metal source or target 31 onto the substrate 3, 13. Forangled deposition, the substrate 3, 13 is inclined by 20 to 70 degrees,such as 30 to 50 degrees, preferably 45 degrees, with respect to thetarget 31. Since the spaces between the ridges 21 are sufficientlysmall, no metal is deposited between the ridges during the angleddeposition. Thus, the tilt angle theta of the substrate should besufficient to prevent metal deposition between the ridges 21. The metalislands 5, 15 may also be deposited by any other suitable angled ornonangled metal deposition method, such as metal organic chemical vapordeposition (MOCVD), molecular beam epitaxy (MBE), sputtering and othersuitable methods.

The ridges 21 may be formed on the substrate 3, 13 by any suitablemethod. Preferably, the ridges are made using lithography. Mostpreferably, the ridges are made using photolithography, as will bedescribed in more detail below. However, the ridges 21 may be made byusing imprint or nanoindentation lithography such as, by stamping atransparent unitary or multilayer substrate with a ridged stamp to forma plurality of ridges and grooves in the transparent substrate.

FIGS. 19A, 19B and 19C illustrate one preferred method of forming theridges in a transparent substrate (i.e., a unitary substrate or amultilayer substrate) 3, 13 or in a layer over the photodetector 203using photolithography. As shown in FIG. 19A, a photoresist layer 41 isformed on the first surface of the substrate 3, 13 (or photodetector203). The term “photoresist layer” includes any suitable positive ornegative photosensitive layer used for semiconductor and othermicrodevice patterning. The photoresist layer 41 is then selectivelyexposed by radiation, such as UV or visible light, or by an electronbeam.

The selective exposure can take place through a mask, by selectivelyscanning a narrow radiation or electron beam across the photoresistlayer 41 or holographically. For example, as shown in FIGS. 19B and 19C,the photoresist layer may be separately exposed holographically for eachcell of the wavelength separation device or the entire layer may beexposed at the same time for a chirped grating pattern.

To perform holographic lithography, a laser beam is split into twobeams. The two beams are then reflected so that they converge togetheronto the photoresist layer 41. Where the two beams converge, aninterference pattern comprised of multiple parallel lines of intenselight is generated. The parallel lines of intense light occur with aparticular periodicity which may be adjusted by changing the incidentbeam angle. Further adjustment of the periodicity may be accomplished bychanges in optics, e.g., changes in the wavelength of the light source,and/or the refractive index of the ambient dielectric adjacent to thephotoresist. Thus, the photoresist is exposed where the two beamsconverge and not exposed where the two beams do not converge. Thelength, Λ, shown in FIG. 19B is equal to the peak wavelength of thesplit laser beams divided by (sinθ₁+sinθ₂), where θ₁ and θ₂ are theangles of the laser beams with the normal to the photoresist surface, asshown in FIG. 19A.

The selective exposure leaves the photoresist layer 41 with exposed andnon-exposed regions. The holographic exposure is preferred because itforms slit shaped exposed and non-exposed regions in the photoresistlayer 41 which can then be used to form slit shaped ridges and groovesin the substrate.

The exposed photoresist layer 41 is then patterned, as shown in FIG.19B. If the photoresist layer 41 is a positive photoresist layer, thenthe exposed regions are removed by a suitable solvent, while leaving theunexposed regions as a photoresist pattern 43 on the substrate 3, 13, asshown in FIG. 19B. If the photoresist layer 41 is a negative photoresistlayer, then the unexposed regions are removed by a suitable solvent,while leaving the exposed regions as a photoresist pattern 43 on thesubstrate 3, 13.

The upper surface of the substrate 3, 13 is then etched to form theridges using the patterned photoresist layer 41 as a mask (i.e., usingthe exposed or non-exposed regions 43 remaining on the substrate as amask). The substrate may be patterned by wet and/or dry etching. Itshould be noted that other intermediate processing steps, such asphotoresist baking, cleaning, etc., may also be added as desired.

Furthermore, if desired, a hardmask layer, such as a silicon nitride,silicon oxide, silicon oxynitride or a metal layer, such as a chromiumlayer, may be added between the photoresist layer 41 and the substrate3, 13 if needed, as shown in FIGS. 19D-19I. As shown in FIGS. 19D and19E, hardmask layer 42, such as a Cr layer, is formed on the substrate3, 13. A photoresist pattern 43 is then formed on the hardmask layer 42by any suitable method, such as the holographic lithography method, asshown in FIG. 19F. The hardmask layer 42 is then patterned using thephotoresist pattern 43 as a mask to form a hardmask pattern 44, and thenthe photoresist pattern 43 is removed, as shown in FIG. 19G. Thesubstrate 3, 13 is then patterned to form the ridges 21 using thehardmask pattern 44 as a mask, as shown in FIG. 19H. The hardmaskpattern 44 is then removed. The metal islands 5 are then selectivelydeposited on the ridges 21, such as by angled deposition, as shown inFIG. 19I.

An example of the parameters of the method described above is asfollows. An about 40 nm thick Cr hardmask layer is deposited on a quartzsubstrate by thermal evaporation. This is followed by HMDS applicationand photoresist spin coating to a thickness of about 100 nm on thehardmask layer. Microposit Photoresist 1805 and Microposit Type PThinner in 1:1 volume ratio is used with a spin speed 5000 rpm. Thephotoresist layer was then subjected to a softbake at 95 degrees Celsiusfor 30 minutes. The photoresist is exposed by holographic lithography. AUV He—Cd laser (325 nm wavelength, 15 mW CW power) is used for theexposure. The photoresist layer is then developed using Microposit 351and DI water in 1:4 volume ratio. The developed (i.e., patterned)photoresist is then subjected to a hardbake at 120 degree Celsius for 30minutes.

The Cr hardmask layer then is etched using the patterned photoresistlayer as a mask. The Cr layer is etched using a reactive ion etching(RIE) system (PlasmaTherm 790 ICP/RIE) in a two step etching process. Instep 1, Cl₂ (20 sccm)+O₂ (10 sccm) at 10 mTorr pressure, RIE power of 25W and ICP power of 100 W for 30 seconds are used. In step 2, Cl₂ (24sccm)+O₂ (6 sccm) at 10 mTorr pressure, RIE power of 10 W and ICP powerof 100 W for 7 minutes are used.

The patterned hardmask layer is then used as a mask to pattern thequartz substrate. The quartz substrate is etched by RIE using CF₄ (37sccm)+O₂ (4 sccm) at 15 mTorr, RIE power of 100 W and ICP power of 150 Wfor 12 minutes. Thereafter, the remaining Cr hardmask is removed bychemical etching with NaOH+K₃Fe(CN)₆+H₂O solution. The Ag islands arethen deposited on the mesa etched substrates using angled deposition.The Ag islands are deposited to various thicknesses using thermalevaporation of Ag source in a base pressure of 10⁻⁵ Torr with a tiltangle of 45 degrees.

FIGS. 20A and 20B illustrate another preferred method of forming theridges in a transparent substrate (i.e., a unitary substrate or amultilayer substrate) 3, 13 or over a photodetector usingphotolithography and a nanopore array. One exemplary method of forming ananopore array is described in Z. Sun and H. K. Kim, Appl. Phys. Lett.,81 (18) (2002) 3458, incorporated by reference herein in its entirety.

First, as shown in FIG. 20A, a photoresist pattern 43 in a shape of agrating is formed on the substrate 3, 13 or over the photodetector 203in the same manner as described above and as illustrated in FIGS.19A-19B. The photoresist pattern may be formed by holographic ornon-holographic lithography. After forming the photoresist pattern 43,the substrate 3, 13 may be etched to transfer the grating pattern to thesubstrate to form ridges 21 illustrated in FIG. 17, after which thephotoresist pattern 43 is removed. Alternatively, the substrate etchingand photoresist pattern removal steps may be omitted.

A metal layer 51 capable of being anodically oxidized is conformallydeposited over the ridges 21, if the ridges are present, or over thephotoresist pattern 43, if the photoresist pattern has not been removed,as shown in FIG. 20B. The conformally deposited metal layer 51 assumesthe grating pattern of the underlying substrate or photoresist pattern,as shown in FIG. 20B. In other words, the metal layer 51 is formed on agrating patterned transparent substrate (i.e., a ridged substrate or apatterned photoresist 43 covered substrate) such that the gratingpattern of the substrate 3, 13 is translated to an upper surface of thefirst metal layer 51.

The metal layer 51 may comprise any suitable metal, such as Al, Ta, Ti,Nb and their alloys, which may be anodically anodized. The metal layer51 may be deposited by any suitable method, such as sputtering, MOCVD,evaporation (thermal or electron beam), MBE, etc. The metal layer 51 mayhave any suitable thickness, such as 100 to 1000 nm, preferably 350-400nm. The corrugation depth in the upper surface of the metal layer 51 ispreferably about the same as the corrugation depth of the substrate orthe photoresist pattern. Preferably, the corrugation depth of the metallayer 51 is about 20 to about 300 nm, such as about 80 to 100 nm.

The metal layer 51 then is oxidized anodically, by any suitable method.For example, an Al layer 51 on a silica substrate 3, 13 may beanodically oxidized in dilute electrolyte (1 H₃PO₄+800 H₂O in volumeratio) at room temperature using a platinum wire as a counter electrode.The anodization is preferably conducted under a constant voltage modefor about 40 minutes. The anodic voltage is chosen such that theexpected pore distance matches the grating period, for example 140 voltsfor a 350 nanometer grating period. In a naturally-formed alumina porearray, the interpore distance is proportional to the anodizationvoltage, i.e., about 2.5 nanometers/volt. The voltage may be varied foranodizing different portions of the metal layer to form pores with avariable period. After anodization, the samples are preferably treatedwith phosphoric acid (diluted with water in a 1:3 volume ratio) for oneto two minutes. FIG. 20C is a electron micrograph of a nanopore array 53grown in the grating patterned aluminum layer 51 when the aluminum layer51 is converted to aluminum oxide by anodic oxidation. The resultingalumina pores exhibit a uniform depth, such as about 100 to 2000 nm,preferably about 300 to 400 nm and the pore bottom has a concave,hemispherical shape with barrier thickness of about 100 to 300 nm, suchas 150 to 200 nm. The preferred pore diameter is about 5 to 100 nm, suchas 5 to 10 nm. The nanopores selectively form in troughs of the gratingpattern in the upper surface of the anodically oxidized metal layer 51.

After forming the nanopore array 53, such as the array shown in FIG.20C, metal islands 5, 15 are selectively grown in the nanopores, asshown in FIG. 20D. One preferred method of selectively growing metalislands inside the nanopores in a metal oxide layer is an electroplatingmethod illustrated in FIG. 20E. The nanopore array 53 is formed on aconductive or a semiconducting substrate 63. The substrate 63 maycomprise a metal layer, such as a metal layer which is not anodicallyoxidized, or a doped semiconductor layer, such as silicon, galliumarsenide or gallium nitride. The substrate 63 may comprise the radiationtransparent substrate 3, 13 used in the devices 1, 11 or the substrate63 may comprise a temporary substrate which is transparent ornon-transparent to radiation. The substrate 63 and array 53 are thenprovided into an electroplating bath 65 containing a liquid metal 67. Apotential difference (i.e., a voltage) is applied between the substrate63 and the array 53. Since the array 53 is thinner in regions 55 belowthe nanopores 57, a voltage gradient exists in these regions 55. Thiscauses the metal 67 from bath 65 to selectively deposit into thenanopores 57. If desired, the electroplating method may be used toselectively fill the nanopores 57 with metal 67 from bath 65. The metal67 may be any metal which exhibits the previously described plasmonenhancement effect and which may be deposited into metal oxide pores byelectrodeposition, such as Ni, Au, Pt and their alloys. Thus, theislands 5, 15 are formed by filling the nanopores 57 with theelectroplated metal 67. By filling nanopores with electroplated metal, ametal island array can be formed that is suitable for an optical device,such as a monochromator and image analyzer applications, and that has astructure complementary to the structure illustrated in FIG. 14.

In an alternative preferred aspect of the present invention, thenanopores 57 are filled only part of the way with the metal 67 duringthe electroplating step. In this case, the metal 67 may be any metalwhich can act as a catalyst for selective metal vapor deposition. Forexample, the metal 67 may be Au. The array 53 with the catalyst metal 67formed on the bottom of the nanopores 57 is then transferred to a metalvapor deposition chamber, such as a chemical vapor deposition chamber.Metal islands 5,15 are then selectively grown on the catalyst metal 67by selective vapor deposition. The metal islands 5, 15 may comprise anymetal which exhibits the previously described plasmon enhancement effectand which may be selectively deposited on a catalyst metal 67, but noton metal oxide walls of the nanopore array 53. For example, this metalmay comprise Al or Ag.

If the nanopore array 53 is formed on a temporary substrate 63, then thetemporary substrate may be removed from the array 53 before or after theformation of the metal islands 5, 15 on the array 53. The temporarysubstrate may be removed by selective etching, polishing or chemicalmechanical polishing of the substrate, by selective etching of a releaselayer (not shown for clarity) located between the temporary substrate 63and the array 53, or by peeling the substrate 63 away from the array 53.In case of peeling, one or more peel apart layers may be located betweenthe substrate 63 and the array 53. The peel apart layer(s) have a lowadhesion and/or strength such that they can be separated mechanicallyfrom each other or from the array and/or the substrate. The transparentsubstrate 3, 13 or the photodetector 203 is then attached to the array53 before or after forming the metal islands 5,15 on the array, on thesame and/or opposite side of the array 53 from where the temporarysubstrate 63 was located.

In an alternative aspect of the present invention, a metal film with aplurality of openings, such as a metal film shown in FIG. 14 is formedby angled deposition of metal on the ridges of a nanopore array. Theangled deposition method is described above and illustrated in FIG. 18.In another alternative aspect of the present invention, a metal layer isdeposited over the nanopore array such that metal extends into thepores, and the metal layer is then chemically mechanically polished oretched back to expose top portions of the nanopore array. The polishingor etch back step leaves discrete metal islands in the nanopores,separated by the metal oxide nanopore array transparent regions.

In another alternative aspect of the present invention, the nanoporearray is formed without first patterning the substrate 3, 13 or formingthe photoresist pattern 43. In this aspect, a metal layer 51, such as anAl, Ta, Ti or Nb layer is deposited on the unpatterned substrate or overan optical device such as a photodetector. Then corrugations are formedin the metal layer 51 by any suitable method. For example, thecorrugations may be formed by selective laser ablation of the metallayer, by nanoindentation or nanoimprinting, or by photolithography(i.e., by forming a photoresist pattern on the metal layer, then etchingthe metal layer using the pattern as a mask and removing the photoresistpattern). Preferably, holographic photolithography is used to patternthe metal layer 51, and a temporary silicon nitride, silicon oxide orsilicon oxynitride hard mask is used between the photoresist and themetal layer 51. Then, the metal layer 51 is anodically oxidized asdescribed above.

FIGS. 21A-D illustrate an alternative method of forming the metalislands using a templated nanopore array. As shown in FIG. 21A, themetal oxide nanopore array 53 on substrate 63 is formed using the methoddescribed above and illustrated in FIGS. 20A-20C. Then, a conformaltemplate material 71 is deposited over the array 63, as shown in FIG.21B. The conformal template material 71 may comprise any material whichcan conformally fill the nanopores 57 of the array 53. For example, theconformal template material 71 may comprise silicon oxide, siliconnitride, a glass heated above its glass transition temperature, a CVDphospho- or a borophosphosilicate glass (PSG or BPSG, respectively), aspin on glass or a polymer material. If desired, the conformal templatematerial may comprise all or part of the transparent substrate 3, 13.

Then, as shown in FIG. 21C, the conformal template material 71 isremoved from the nanopore array 53. The conformal template material 71contains ridges 73 which previously extended into the nanopores 57 ofthe array. Then, the metal islands 5,15 are selectively deposited intothe pores 75 between the ridges 73 of the conformal template material 71using the electroplating method or on the ridges 73 using angleddeposition method as described above. If the conformal template material71 is the transparent substrate 3, 13 material, then the process stopsat this point. If the conformal template material 71 is not thetransparent substrate 3, 13, then the conformal template material 71 isseparated from the metal islands 5, 15 by any suitable method, such asselective etching, polishing or chemical mechanical polishing. The metalislands 5, 15 are attached to the transparent substrate 3, 13 before orafter removing material 71.

FIGS. 22A and 22B illustrate an alternative method of forming the metalislands 5, 15 without using ridges on a substrate and without using ananopore array. In this method, a metal layer 81 is formed on thesubstrate 3, 13, as shown in FIG. 22A. The substrate 3, 13 may containfeatures on its upper surface or it may contain a flat upper surface.The metal layer 81 is then patterned into a plurality of metal islands5, 15 as shown in FIG. 22B. The metal layer 81 may be patternedlithographically as described previously. Thus, a photoresist layer 41is formed on a first surface of the metal layer 81. The photoresistlayer is selectively exposed to form exposed and non-exposed regions.The exposed photoresist layer is patterned into pattern 43 and the metallayer is etched into the plurality of islands 5, 15 using the patternedphotoresist layer as a mask.

The photoresist layer may be exposed holographically ornon-holographically. If desired, an optional, temporary hardmask layerdescribed above may be formed between the metal layer 81 and thephotoresist. Alternatively, the metal layer may be patterned byselective laser ablation or other non-photolithographic methods insteadof by photolithography.

FIGS. 23A, 23B and 23C illustrate an alternative lift off method offorming the metal islands 5, 15. This method also does not require usingridges on a substrate or a nanopore array. In this method, a photoresistlayer 41 is formed on the substrate 3, 13 or over the photodetector 203as shown in FIG. 23A. The substrate 3, 13, may contain features on itsupper surface or it may contain a flat upper surface. The photoresistlayer is selectively exposed to form exposed and non-exposed regions.The photoresist layer may be exposed holographically ornon-holographically.

The exposed photoresist layer 41 then is patterned to form a photoresistpattern 43, exposing portion of the upper surface of the substrate 3,13. As shown in FIG. 23B, a metal layer 81 is formed over thephotoresist pattern 43 and over exposed portions of the upper surface ofthe substrate 3, 13.

As shown in FIG. 23C, the photoresist pattern 43 is then lifted off,such as by selective etching or other suitable lift off techniques.Portions of the metal layer 81 located on the photoresist pattern 43 arelifted off with the pattern 43 to leave a plurality of metal islands 5,15 on the upper surface of the substrate 3, 13.

The foregoing description of the invention has been presented forpurposes of illustration and description. It is not intended to beexhaustive or to limit the invention to the precise form disclosed, andmodifications and variations are possible in light of the aboveteachings or may be acquired from practice of the invention. Thedrawings and description were chosen in order to explain the principlesof the invention and its practical application. It is intended that thescope of the invention be defined by the claims appended hereto, andtheir equivalents.

1. A nano-optic device comprising a plurality of subwavelength aperturesin a metal film or between metal islands, wherein the device is adaptedto shape a radiation beam transmitted there through and wherein: eachaperture has a width that is less than a first peak wavelength ofincident radiation to be provided onto the film or islands; and themetal film or islands are configured such that the incident radiation isresonant with at least one plasmon mode on the metal film or metalislands, thereby enhancing transmission of radiation having at least onesecond peak wavelength through the apertures.
 2. The device of claim 1,wherein shaping the radiation beam comprises at least one of beamfocusing, beam bending and beam collimating.
 3. The device of claim 1,wherein the device comprises a lens which is adapted to focus theradiation beam to a spot that is the same as or smaller than a peakwavelength of the radiation beam.
 4. The device of claim 3, wherein thespot size ranges from 10 nm to 800 nm.
 5. The device of claim 3, whereinthe lens comprises a metal film having a convex profile such that theapertures have a different depth in a half elliptical profile.
 6. Thedevice of claim 3, wherein the lens comprises a flat metal filmcontaining the apertures and a dielectric lens mounted thereon.
 7. Thedevice of claim 1, wherein the device is a beam bending devicecomprising a metal film containing different transparent refractiveindex materials in different apertures, above different apertures or inand above different apertures.
 8. The device of claim 1, wherein thedevice is a beam bending device comprising a metal film containing, ormetal islands separated by, apertures of different width.
 9. The deviceof claim 1, wherein the device further comprises at least one of aspatial, wavelength and polarization filter.
 10. The device of claim 1,wherein the device is used together with at least one of a spatial,wavelength and polarization filter.
 11. The device of claim 1, whereineach aperture serves as a dipole source radiating optical power at anexit surface of the metal film or islands.
 12. The device of claim 11,wherein the dipole elements are designed to have a predetermined phaserelationship among them controlled by at least one of a path lengthdifference and an effective index difference in the aperture regions.13. The device of claim 1, wherein the aperture width is between 15 nmand 700 nm.
 14. A nano-optic device comprising a plurality ofsubwavelength apertures in a metal film or between metal islands,wherein the device is adapted to shape a radiation beam transmittedthere through and wherein: the device comprises a lens which is adaptedto focus the radiation beam to a spot that is the same as or smallerthan a peak wavelength of the radiation beam; and the lens comprises: i)a metal film or metal islands having a convex profile such that theapertures have a different depth in a half elliptical profile; or ii) adielectric lens mounted on a flat metal film containing the apertures oron flat metal islands separated by the apertures.
 15. The device ofclaim 14, wherein the spot size ranges from 10 nm to 800 nm and theaperture width is between 15 nm and 700 nm.
 16. The device of claim 14,wherein: each aperture has a width that is less than a first peakwavelength of incident radiation to be provided onto the film orislands; and the metal film or islands are configured such that theincident radiation is resonant with at least one plasmon mode on themetal film or metal islands, thereby enhancing transmission of radiationhaving at least one second peak wavelength through the apertures. 17.The device of claim 14, wherein the device further comprises at leastone of a spatial, wavelength and polarization filter or the device isused together with at least one of a spatial, wavelength andpolarization filter.
 18. The device of claim 14, wherein: each apertureserves as a dipole source radiating optical power at an exit surface ofthe metal film or islands; and the dipole elements are designed to havea predetermined phase relationship among them controlled by at least oneof a path length difference and an effective index difference in theaperture regions.
 19. The device of claim 14, wherein the lens comprisesthe metal film having the convex profile such that the apertures havethe different depth in the half elliptical profile.
 20. The device ofclaim 14, wherein the lens comprises the metal islands having the convexprofile such that the apertures have the different depth in the halfelliptical profile.
 21. The device of claim 14, wherein the lenscomprises the dielectric lens mounted on the flat metal film containingthe apertures.
 22. The device of claim 14, wherein the lens comprisesthe dielectric lens mounted on the flat metal islands separated by theapertures.
 23. A nano-optic device comprising a plurality ofsubwavelength apertures in a metal film or between metal islands,wherein the device is adapted to shape a radiation beam transmittedthere through and the device comprises a beam bending device wherein: i)the metal film contains apertures of different width; or ii) the metalislands are separated by apertures of different width; or iii) differenttransparent refractive index materials are located in differentapertures, above different apertures or in and above differentapertures.
 24. The device of claim 23, wherein the aperture width isbetween 15 nm and 700 nm.
 25. The device of claim 23, wherein: eachaperture has a width that is less than a first peak wavelength ofincident radiation to be provided onto the film or islands; and themetal film or islands are configured such that the incident radiation isresonant with at least one plasmon mode on the metal film or metalislands, thereby enhancing transmission of radiation having at least onesecond peak wavelength through the apertures.
 26. The device of claim23, wherein the device further comprises at least one of a spatial,wavelength and polarization filter or the device is used together withat least one of a spatial, wavelength and polarization filter.
 27. Thedevice of claim 23, wherein: each aperture serves as a dipole sourceradiating optical power at an exit surface of the metal film or islands;and the dipole elements are designed to have a predetermined phaserelationship among them controlled by at least one of a path lengthdifference and an effective index difference in the aperture regions.28. The device of claim 23, wherein the device comprises the metal filmcontaining apertures of different width.
 29. The device of claim 23,wherein the device comprises the metal islands separated by apertures ofdifferent width.
 30. The device of claim 23, wherein the devicecomprises different transparent refractive index materials located indifferent apertures, above different apertures, or in and abovedifferent apertures.
 31. A nano-optic device comprising a plurality ofsubwavelength apertures in a flat metal film or between metal islands,wherein the device is adapted to shape a radiation beam transmittedthere through and wherein the device further comprises at least one of aspatial, wavelength and polarization filter.
 32. A nano-optic devicecomprising a plurality of subwavelength apertures in a flat metal filmor between metal islands, wherein the device is adapted to shape aradiation beam transmitted there through and wherein: each apertureserves as a dipole source radiating optical power at an exit surface ofthe metal film or islands; and the dipole elements are designed to havea predetermined phase relationship among them controlled by at least oneof a path length difference and an effective index difference in theaperture regions.